Photographer Karine Aigner has documented two scientific studies making use of nanotags – tiny radio transmitters attached to birds – that are helping to lift the lid on the complexities of bird migration.

In a world that is becoming increasingly challenging for migratory birds, nanotag studies could have broad implications for the long-term survival of hundreds of species…

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On Block Island, off the coast of the U.S. state of Rhode Island, biologist Clara Cooper-Mullin is studying the migration behaviour of birds that use the island as a rest stop on the way to their wintering grounds. Cooper-Mullin and her team are interested in how fuel stores and antioxidant capacity affect the length of time a bird chooses to spend on a stopover – a decision that can make or break its overall migration success. To find out, the team sets up mist nets to catch birds without hurting them.

The team sets up mist nets to catch birds without hurting them. The nets are checked every hour, and the birds carefully disentangled. There’s no way of knowing which birds will be captured, or how many.

Each test bird is fed a carefully-controlled diet, then fitted with a nanotag before release to assess whe­ther their diets affect how long they stay on Block Island, and whether their departure is in a seasonally appropriate (southerly) direction. The nanotags, tiny and weighing only a fraction of a gram, are radio transmit­ters that send out a uni­quely coded pulse. The signals are received by radio towers, which can pick up the pulses of any tagged birds flying within range. The tracking data will later reveal that three out of four of her test species who received extra fat left Block Island sooner than those that didn’t.

1,900 kilometres south of Block Island lies The Bahamas. This coral-based archipe­lago is the site of another study, which is also using nanotags to better unders­tand bird migration. This time, the biologist is Nathan Cooper (no relation to Cooper-Mullin) and the bird is the Kirtland’s warbler. Cooper’s study is ground-breaking. Thanks to the nanotags he’s fitting to birds on Cat Island – one of the Bahamian wintering spots for this species – Cooper’s team are able to track the very same individuals thousands of miles north to their breeding range in Michigan. With both winter and summer data in hand, he can begin to answer the question of how a warbler’s fat stores and muscle mass on Cat Island affect its departure from The Bahamas, as well as the speed of its migra­tion and the subsequent timing of its arrival in Michigan.

More and more, scientists are finding that winter conditions – not just the quality of the breeding habitat in the north – are critical to the nesting success of migratory birds. Research has already revealed a strong connec­tion between rainfall in the Kirtland’s warblers’ winte­ring grounds and their like­lihood of survival through the spring migration. In short, less rainfall in The Bahamas means fewer warblers in Michigan. Given that the tropical regions on which hundreds of millions of migratory birds depend are already warming and drying—a trend that is expected to accelerate— Cooper’s research with Kirtland’s warblers has broad implications for hundreds of species.

Nanotag studies are paving the way to a greater understanding of migratory bird ecology. Using the miniaturised radio transmitters, scien­tists such as Nathan Coo­per and Clara Cooper-Mul­lin are beginning to tease apart complex questions about how a bird’s diet, fat stores and muscle mass affect when it departs from its wintering grounds, and how long it spends at stopover sites. Birds as small as warblers can carry one without impairing their ability to fly, find food, or evade predators – and the data gathered can help scientists identify conservation priorities, deploying efforts and funds where they’re most needed.

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